(1) Field of the Invention
The present invention relates to a method for producing a hydrogen-absorbing alloy electrode including a hydrogen-absorbing alloy that reversibly absorbs and desorbs hydrogen at atmospheric pressure, and more particularly to treating the surface of the hydrogen-absorbing alloy.
(2) Description of the Related Art
Lead storage cells and nickel-cadmium storage cells have been widely used so far; however, metal-hydrogen alkaline storage cells are now attracting attention. A metal-hydrogen alkaline storage cell is composed of a negative electrode using a hydrogen-absorbing alloy as an active material and a positive electrode using a metallic oxide such as a nickel hydroxide as an active material. These metal-hydrogen alkaline storage cells have advantages of being lighter in weight, larger in capacity, and higher in energy density than these conventional storage cells.
The hydrogen-absorbing alloy used in a metal-hydrogen alkaline storage cell is subjected to various surface treatments in order to improve the cell characteristics.
For example, Japanese Patent Publication No. 4-79474 discloses the use of an alkaline aqueous solution for a surface treatment of the hydrogen-absorbing alloy. The surface treatment realizes the formation of a nickel-rich layer over the surface of the hydrogen-absorbing alloy. The nickel-rich layer accelerates smooth absorption of oxygen gas generated from the positive electrode during overcharge because nickel works as a catalyzer in a reducing reaction of oxygen. Consequently, the rise in cell internal pressure is restricted and charge/discharge cycle characteristics are improved.
Japanese Laid-open Patent Application No. 3-98259 discloses the use of hot water of 60.degree. C. or higher for a surface treatment of a hydrogen-absorbing alloy. This method also realizes the formation of a nickel rich layer with the same effects as the above case.
However, these surface treatments to form a nickel rich layer have an undesirable side effect of producing hydroxide. The hydroxide increases the contact resistance among hydrogen-absorbing alloy particles or decreases the activity of the hydrogen-absorbing alloys, thereby deteriorating the high rate charge/discharge characteristics and low temperature discharge characteristics in the initial charge/discharge cycle.
The following methods have been proposed for improving the high rate charge/discharge characteristics and low temperature discharge characteristics in the initial charge/discharge cycle.
1) Japanese Laid-open Patent Application No. 3-152868: A fine oxide layer which obstructs the initial activation of a hydrogen-absorbing alloy is removed through an acid treatment before an alkaline treatment is applied to the alloy. As a result, a hydrogen-absorbing alloy electrode with both excellent high rate charge/discharge characteristics and gas absorption characteristics is obtained. PA1 2) Japanese Laid-open Patent Application No. 1-132048: An acid treatment is applied to a hydrogen-absorbing alloy, and a sintered electrode with excellent high rate charge/discharge characteristics and charge/discharge cycle characteristics is produced by using the alloy. PA1 .theta.: Bragg's diffraction
However, according to the former method, the alkaline treatment results in the formation of hydroxide, and the low temperature discharge characteristics are not satisfactory. The latter method, on the other hand, has a problem that the complicated producing process of the sintered electrode leads to low productivity.
If the acid treatment is applied to a non-sintered electrode, acid melts the alloy surface, thereby generating hydrogen. The generated hydrogen is re-absorbed by the alloy, and the alloy has cracks on its surface during the re-absorption. As a result, new active surfaces are formed which improve the activity of the alloy. If the cracks grow larger, however, the alloy may be deteriorated through oxidization with the progress of charge/discharge cycles. The deterioration of the alloy lowers the charge/discharge efficiency and the charge/discharge cycle characteristics.